1. Field of the Invention
The present invention relates to glass ceramics and, more particularly, to a glass ceramic for use in biological applications such as prostheses including artificial crowns, artificial dental roots and artificial bones.
2. Brief Description of the Prior Art
Several different glass ceramics each containing calcium phosphate crystals for use as biological materials have heretofore been reported. These glass ceramics can be roughly classified into the following three types. The first type is based on CaO and P.sub.2 O.sub.5 and contains CaO.P.sub.2 O.sub.5 as a crystal phase as described in Japanese Unexamined Patent Application No. 73019/1976. The second type is based on MgO, SiO.sub.2, CaO and P.sub.2 O.sub.5 and contains apatite crystals and crystals of silicates such as wollastonite, diopside, mica, etc. as crystal phases as described in Japanese Unexamined Patent Application No. 191252/1982. The third type is based on CaO, P.sub.2 O.sub.5, SiO.sub.2, Al.sub.2 O.sub.3 and B.sub.2 O.sub.3 and contains tricalcium phosphate as a dominant crystal phase as described in Japanese Unexamined Patent Application No. 26743/1981.
The advantage of using a glass ceramic as a biological material is that the glass ceramic is highly biocompatible due to the presence of calcium phosphate crystals in it. Another advantage is, being a type of glass, it can be cast from a melt and, then, crystallized to directly give a product having a desired shape with high dimensional accuracy. However, in regard to the first mentioned type of glass ceramic based on CaO and P.sub.2 O.sub.5, it is well known that a stable glass is available only in the region of not more than 56 mole percent of CaO, that is to say in the Ca/P range of &lt;0.64. [Sumio Sakka et al (ed.): Glass Handbook, Asakura Shoten, p. 882 (1975)]. For this reason, the product glass ceramic is inadequate in water resistance and hence tends to release its components in vivo to lower the pH of the local humor to manifest an adverse effect on the living body.
Improvements in water resistance may be achieved by increasing the Ca/P ratio of glass but because of the above-mentioned glass-forming region, it is actually difficult to obtain a glass ceramic having a sufficiently high water resistance unless other glass network former such as SiO.sub.2 and B.sub.2 O.sub.3 are present together with the phosphate component.
Glass ceramics of the above-mentioned second and third types are the products developed to meet the above requirement. However, the fusing temperature of the second type of glass is as high as 1,400.degree.-1,500.degree. C. and can hardly been cast with the conventional technologies applicable to the metals. Because the silicate crystal has a strong tendency of inducing surface devitrification, it is very difficult to directly crystallize pre-cast glass bodies having this type of composition. Therefore, it is necessary to pulverize the glass, shape the glass powder into a desirable article by pressing or conventional method for ceramics and, then sinter to crystallize the green body by post-heating. This entails difficulties in regard to the maintenance of dimensional accuracy.
The third type of glass can be fused at a somewhat lower temperature than the second type of glass but its drawback is that a majority of precipitated crystals are tricalcium phosphate crystals with a Ca/P ratio of 1.5. However, in order to overcome the above-mentioned problem of in vivo release of glass components, it is desirable that apatite crystals with a higher Ca/P ratio than 1.5 is precipitated.